You sounds like us, planning one city at a time.

Here are our plans (this is mostly from my benefit but might give others ideas)
London - Thursday - find Melbourn House hotel in city, visit Tower Bridge, Tower of London whatever else we feel up for after the long over-night flight
London - Friday - HOHO tour of the city
while keeping an eye out for Andrew.
London/Dover - pick up rental car in London near hotel, drive to Leeds Castle. Try to remember to drive on the wrong side of the road. If we think we have time, stop by Canterbury Cathedral. Continue on to Dover, drop off rental car at port and board ship.
Oslo - since we've been to Oslo before and hit all the 'mandatory' tourist stops, we'll do some shopping for Norwegian sweaters, eat some local food, visit the park with all the sculpture's and maybe tour Aukershus again. Might also do the Fram tour again.
Copenhagen - walk to Trivoli park and spend a few hrs. Rest of the day is still to be determined (TBD).
Germany - pick up rental car in Rostock. Visit local sites. Drive out to Wismar per Frommer's book. Stop at some small towns to visit the rural Germans.
St Petersburg - two day private tour of SPR,
DCL Folkloric excursion
Helsinki - Walking tour per Frommers, might rent bikes. Mostly still TBD and need to do some more planning/research.
Stockholm - HOHO bus, Vasa museum. Very short port day now that we are leaving at 1 pm. BTW, got a card in the mail yesterday from DCL saying the port authority is making them leave early. My contact at DCL says they are planning "something special" for that afternoon since we are getting robbed of the port time.
Dover - pick up rental car at port. Tour Dover castle, drive to Canterbury cathedral, say a quick "Our Father" and then head to Heathrow for 6 pm flight to Newark.
~Mike